That means that any file dropped into the folder will take on the folder's owning group.
For example: Suppose you have a folder called "shared" which belongs to user "intrpc" and group "users", and you (as user "initrpc") drop a file into it. As a result, the file will be belong to user "intrpc" and group "users", regardless of "initrpc"'s primary group.
On most systems, if a directory's set-group-ID bit is set, newly created subfiles inherit the same group as the directory, and newly created subdirectories inherit the set-group-ID bit of the parent directory.
You can read about it here.
Why is the letter uppercase (from the link you gave)?
setgid has no effect if the group does not have execute permissions.
setgid is represented with a lower-case "s" in the output of ls. In
cases where it has no effect it is represented with an upper-case "S".